A silver halide photographic material comprises a silver halide emulsion layer provided on a support. The silver halide emulsion is usually chemically sensitized to obtain a desired sensitivity or gradation. Examples of the chemical sensitizer include a sulfur sensitizer, a selenium sensitizer, a tellurium sensitizer, a noble metal (such as gold) sensitizer, a reduction sensitizer and a combination thereof.
For several years there has been a growing demand for improvement of silver halide photography. The recent photographic material requires very high sensitivity. Further, an improvement of the graininess and the sharpness has been required with respect to the formed image. A rapid image forming process such as a quick development process is also required. The sensitization and the sensitizer have been improved to meet these requirements.
Tellurium sensitizers are disclosed in U.S. Pat. Nos. 1,574,944, 1,623,499, 3,320,069, 3,531,289, 3,655,394, 3,772,031, 4,704,349, British Patents No. 235,211, 1,121,496, 1,295,462, 1,396,696, Canadian Patent No. 800,958, Japanese Patent Provisional Publications No. 53(1978)-57817, No. 61(1986)-20940 and No. 61(1986)-67845.
Some tellurium compounds have been used as additives of a photographic material such as antifogging agents. The tellurium compounds other then the sensitizers are disclosed in U.S. Pat. Nos. 4,607,000, 4,607,001, Japanese Patent Provisional Publications No. 62(1987)-234153, No. 63(1988)-65438, No. 2(1990)-118566, No. 2(1990)-140736, No. 2(1990)-158730 and No. 3(1991)-91735.
The tellurium sensitizers such as colloidal tellurium and potassium telluride disclosed in Canadian Patent No. 800,958 are excellent in sensitivity, compared with conventional sulfur sensitizers. However, the colloidal tellurium is poor in reproducibility, since the quality of the colloidal tellurium is greatly influenced by the conditions in synthesis. For example, the colloidal tellurium is prepared by using a strong reducing agent such as stannous chloride. A trace amount of the reducing agent which remains after synthesis influences the quality of the colloidal tellurium. Further, potassium telluride is also poor in reproducibility. Moreover, it is rather difficult to handle potassium telluride, since it is not a stable compound.
Tellurium compounds are generally not stable. Accordingly, it is difficult to reproduce a photographic property when a tellurium compound is used in a photographic material. A tellurium sensitizer demands an excellent reproducibility and stability.
By the way, a silver halide photographic emulsion is usually spectrally sensitized with a sensitizing dye. Silver halide inherently has a blue sensitivity. The spectral sensitivity is changed by a sensitizing dye to the other light regions such as green, red or infrared regions.
The spectral sensitivity is preferably as strong as possible. Even if a large amount of the sensitizing dye is used, the spectral sensitivity is not so increased as is expected. This phenomenon is called inherent desensitization. It is particularly remarkable within a long wavelength region. The inherent desensitization is a result of development inhibition or latent image diffusion caused by the dye and invalidity of photoelectron or latent image bleaching caused by a positive hole of the dye.
If a large amount of the sensitizing dye is used, the sensitivity of a photographic material is not stable. The sensitivity is usually decreased, while the photographic material is preserved.
Therefore, an improvement is required to increase steadily the spectral sensitivity of a photographic material.
Furthermore, a rapid image forming process such as a quick development process has recently been particularly required.